As a result of errors in the position computed by inertial sensors and polar discontinuities in latitude and longitude, there can be a disparity in the north reference between the various inertial sensors aboard a vehicle which is near a pole. The north reference disparity among the inertial sensors prevents a direct and meaningful determination of velocity/track reasonableness (by comparing velocity data among the various sensors) from being made. Also, a simple or weighted average of position and velocity data output by multiple inertial sensors becomes very error prone as the aircraft approaches a pole.
In addition, any previously computed and stored inertial position bias (latitude and longitude errors) becomes subject to error when the north reference of the vehicle changes as it progresses across the globe. A changing north reference causes the stored inertial position bias to appear to rotate resulting in position accuracy degradation if this phenomena is not accounted for. At higher latitudes the error induced as a result of a changing north reference can become so severe that it is not desirable to use a previously computed and stored inertial position bias in the form of latitude and longitude errors.
A prior art system is described in U.S. Pat. No. 3,214,575, issued Oct. 26, 1965. Related patents include U.S. Pat. Nos. 3,232,103, issued Feb. 1, 1966, 4,405,986, issued Sept. 20, 1983, 4,520,445, issued May 28, 1985, and 4,675,820, issued June 23, 1987. Other background patents include U.S. Pat. Nos. 3,339,062, issued Aug. 29, 1967, 3,940,597, issued Feb. 24, 1976, 4,168,524, issued Sept. 18, 1979, and 4,442,491, issued Apr. 10, 1984.
Although the prior art systems have one thing in common, i.e., they all utilize an algorithm which may be employed in a navigation device to compute navigation quantities (position, velocity, vehicle attitude, etc.), none of them are concerned with the problem of multiple independently operating navigation units wherein each unit provides separate complete navigational quantities for comparison in the manner of the present invention.
As a result of independent operation and uncontrollable manufacturing differences, the navigational quantities estimated by each unit (i.e., position and velocity) would drift away from the actual values in an unknown and random way. The problem then becomes one of determining which of the units if any were functioning properly and then to combine data from all of the functioning units in such a way that the best estimate of the actual navigational quantities could be obtained. This estimate would provide greater accuracy than any of the units operating alone. None of the prior art systems accomplish this result in the manner of the present invention.
There are certain additional disadvantages of the prior art that will become evident which include:
1. At high and low latitudes, storing the inertial position bias as latitude and longitude errors is not desirable since the changing north reference of the vehicle will cause the values stored to be incorrect if they are not continuously updated (which is often impossible).
2. Washing out the position bias degrades aircraft position accuracy.
3. The computed inertial position and velocity are less accurate if inertial position and velocity data is not averaged.
4. There is no way to detect excessive drift in an inertial reference unit if comparison tests with other inertial positions and velocities are not performed.
5. The reduction in position accuracy as a result of washing out an inertial position bias when approaching the pole can create problems for a system which also computes a position(s) by means other than inertial sensors. Typically, the computer noninertial position must be compared to a reference position (usually the inertial position plus the inertial position bias) to determine if data from the noninertial sensor (or sensors) is reasonable enough to be used. If the accuracy of the reference position is reduced significantly by the loss of the inertial position bias, then determination of whether a noninertial position is reasonably correct can be severely complicated.